1. Field of the Invention
The invention relates to PVD hard coating materials for engineering components used in a hot environment, especially for tools for the coolant-free and lubricant-free machining of materials.
The coolant-free and lubricant-free machining of materials, in particular of metals like die steels or aluminium alloys, is increasingly gaining in significance, in particular in the automotive industry.
2. Description of the Prior Art
TiN and TiCN layers deposited by the known PVD and CVD methods cannot satisfy this object satisfactorily because both begin to oxidize at operating temperatures beyond 400.degree. C. and thus considerable wear arises, in particular at the cutting edges. This is because the oxides which are formed are very brittle and spall off from the TiN or TiCN coating due to their low bond strength and different thermal expansion. This leads to a continuous and considerable reduction of the layer thickness in the edge region during cutting operations at elevated temperatures. Analogous effects can also arise with forming tools.
It has been found that TiAlN applied by the PVD process as a hard material coating is much better suited to resisting the oxidation processes. It has been reported that the temperature at which oxidation sets in can be increased to 700 to 800.degree. C when the hard material layer consists, for example, of 50 at % Ti and of 50 at % Al.(W.-D. Munz, J. Vac. Sci. Technol., A4 (6) (1986) 2117).
Even higher Al contents have been reported, i.e. Al contents of up to 70 at % of the respective metal content. It is known from the field of turbine blade coating that the addition of yttrium increases the oxidation resistance of superalloys such as CoCrAlY or of thermally insulating layers of zirconium oxide.
In the hard material coating field a layer of TiYN has admittedly been reported with a Y-content of more than 10 at %; however, this layer proved to be unsuitable for industrial use, mainly because of its brittleness. (J. R. Roos, J. P. Celis, E. Vancoille, H. Veltrop, S. Boelens, F. Jungblut, J. Ebberink, H. Homberg, Thin Solid Films, 193/194 (1990) 1542).